The present invention relates to radio frequency (“RF”) heating. In particular, the present invention relates to an advantageous and efficient apparatus and method for heating substances of varying conductivities.
RF heating can be used in a variety of applications. For example, oil well core samples can be heated using RF energy. These core samples, however, can vary greatly in conductivity, and therefore respond differently to various types of heating. Dielectric heating is efficient and preferable for samples having a low conductivity. Samples with higher conductivity are best heated by inductive heating. Medical diathermy, or the use of heat to destroy abnormal or unwanted cells, is another application that may utilize RF heating.
RF heating is a versatile process for suitable for many materials as different RF energies may be used. There can be electric fields E, magnetic fields H, and or electric currents I introduced by the RF heating applicator. Linear applicators, such as a straight wire dipole emphasize strong radial near E fields by divergence of current I. Circular applicators, such as a wire loop emphasize strong radial H fields by curl of current I. Hybrid applicator forms may include the helix and spiral to produce both strong E and H fields. Uninsulated RF heating applicators may act as electrodes to introduce electric currents I in the media.
Parallel linear conductors form an antenna in U.S. Pat. No. 2,283,914, entitled “Antenna” to P. S. Carter. Now widely known as the folded dipole antenna, the antenna uses equal direction current flows in the thin wires and a voltage summing action to bring the driving impedance to a higher value. The folded dipole antenna did not, however, include aspects of: antiparallel current flow (opposite current directions or senses), operation with open terminals at one end, induction coupling to a separate feed structure, or capacitor loading. The folded dipole antenna is useful for operation at sizes of about ½ wavelength and above.
U.S. Pat. No. 2,507,528 entitled “Antenna” to A. G. Kandoian describes antiparallel (equal but opposite direction) currents flowing on the opposite edges of a slot in a conductive plate. Horizontal polarization was realized from a vertically oriented slot.
RF heating may operate by near fields or far fields. Near fields are strong reactive energies that circulate near RF heating applicators. Far fields may comprise radio waves at a distance from the applicator. Both near and far fields are useful for RF heating, and many tradeoffs are possible. For instance, near fields may be more useful for low frequencies, when the applicator is small in size, and for conductive materials. Far fields may be preferred for heating at a distance and for heating low conductivity materials.